The invention relates to devices and methods for administering metered amounts or doses of an injectable product, in particular to devices and methods involving the type of injection devices known as injection pens for, e.g., administering insulin.
A device of the type to which the invention relates is known from the specification of document WO 97/36626. The device has a housing incorporating a reservoir for the product. A plunger is accommodated in the reservoir, which forces the product out of the reservoir through an outlet of the reservoir when pushed in a forward direction. A toothed rack serves as a plunger rod and pushes the plunger in the forward direction. Also disposed in the housing is a drive member which is slidable relative to the housing in the forward direction and in reverse, which drives the toothed rack with it when pushed in the forward direction. To this end, the drive member has drivers which engage in rows of teeth of the toothed rack. In order to set the quantity of product to be administered with one stroke, i.e., by operating a dose metering unit, the drive member is manually pulled back from a forward position in the direction opposite the forward direction by a set length defining the dose. As this happens, the drivers of the drive member slide across the teeth of the rows of teeth of the toothed rack and elastically give way. The toothed rack is prevented from being pushed back by locking means which are secured so as to prevent any sliding relative to the housing. The locking means co-operate with one of the rows of teeth of the toothed rack so that the locking means prevent the toothed rack from sliding in the direction opposite the forward direction. They allow the toothed rack to slide in the forward direction due to elastic flexibility. When the drive button is operated, the set path length defining the dose is travelled by the toothed rack and plunger due to the drive member so that the set dose is dispensed through the outlet of the reservoir.
The specification of EP 0498737 discloses a dose metering unit for an injection device, incorporating a mechanism for setting the dose and administering the product. The mechanism moves a sleeve between two settings. In the position intended as a means of setting the dose, the length by which a plunger rod will be displaced is set and hence also the dose. Accordingly, locating jaws connected to a drive member are not engaged with the plunger rod. When the sleeve is turned into the second position, the injection device is in an injection-ready state. Accordingly, the jaws engage between teeth of a row of teeth of the plunger rod, pushing the drive member. As the drive member is pushed in, the jaws engage directly in the row of teeth. The injection device also has a return lock for the plunger rod. To this end, two claws are provided in the interior of the injection device, which do not locate in the plunger rod in the released state. In this state, the plunger rod can be displaced backwards and forwards. When an ampoule is turned in the injection device, an edge provided within a housing sleeve for the ampoule opposes the claws and pushes their teeth into the row of teeth. However, because of the layout of the teeth of the toothed rack and the claws, the locating action is such that the plunger rod is able to move in the forward direction but not in an opposite direction.
U.S. Pat. No. 6,228,067 describes an injection device which also has a dose metering and drive unit. When two housing regions are turned towards one another, an axial displacement of a metering element is triggered in one of the housing parts, thereby enabling a specific dose to be set. In this instance, the drive unit is initially engaged with a toothed rack, which pushes the plunger forward to dispense the dose. When an operating element is pulled back, the toothed rack is pulled back in the direction opposite the forward direction but the plunger remains in its position because locating projections engage between teeth of a row of teeth of the toothed rack. The locating projections are disposed on an extension of an operating button and when located are biased. When the operating button is pulled out, the plunger rod is displaced in the direction opposite the forward direction until cams provided on the extensions opposite the projections locate in recesses of the housing internal face due to the biasing action. As a result, the projections are pulled out of the intermediate spaces of the row of teeth of the toothed rack, so that they are released. A spring moves them in the forward direction until they sit against the plunger. To administer an injection, the operating button is moved in the forward direction, causing the cams of the extensions to be forced out of the recesses in the internal face of the housing, thereby pushing the projections between the teeth of the row of teeth so that the operating button engages with the plunger rod again.
The path length or travel defining the dose in the injection devices known from the prior art is generally very short, which means that the operating button also has to be moved by this short path length in order to administer an injection. As soon as the operating button is moved in the forward direction, the different drive mechanisms engage on the plunger rod and move it in the forward direction, thereby dispensing the desired dose. Due to the fact that there is little room left for the operating button to move when administering an injection, it is often difficult for the user to see and/or determine in what position or what state the injection device is at any one time. To ensure that the entire path length determining the dose is covered, it is often necessary to exert an unnecessarily high force on the operating button or on the entire injection device. This can lead to uncertainties in terms of guiding the device, and/or may lead to excessive wear of the material.